The SI Multi-Function End Plate and Anti-Rotation Castle Nut are designed to modernize and improve the often overlooked castle nut and endplate system. While the traditional endplate and castle nut work very well in securing your receiver extension, peening the castle nut is not ideal for rebuilds. The MFEP and ACN include an anti-rotation QD socket or sling loops. Instead of peening the endplate to secure the castle nut, a set screw performs this task, allowing for multiple uses as you upgrade or update your build.